Communication Functions of CPU Unit
1.3.3 MODBUS TCP Master/Slave Communication
Overview of function
• This is used for communicating with other devices that support the MODBUS TCP protocol.
• In master communication, communication is performed when the master unit sends
instructions (command messages) to slave units and the slave unit returns responses
(response messages) according to the instructions. Messages in accordance with the
protocol are automatically generated by PLC. In the user program, reading and writing can
be done simply by specifying the station no. and memory address and executing
SEND/RECV commands.
• Slave communication is performed when the higher device connected to PLC has the
sending right, and sends commands, and PLC returns responses. In slave communication,
PLC responds automatically, so no program concerning communication is necessary on the
PLC side.
• The data size that can be sent or received in a single communication is up to 127 words for
register transmission and 2040 bit for bit transmission.
FP7 CPU
MODBUS-TCP
supporting device
Ethernet LAN
IP XXXXXX
IP XXXXXX
Send/receive data using
MODBUS-TCP
Command
Master
communication
Slave
communication
Response
Command
Response
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